Technical Field
The present invention relates to a method for operating at least one wind turbine. The present invention relates further to a wind turbine and it relates to a wind farm. In addition, it relates to a method for operating a wind farm.
Description of the Related Art
Wind turbines are generally known, they transform, in particular by means of a rotor, which rotates around an essentially horizontally arranged axis, mechanical wind power into electrical energy, for which, in addition, an electric generator is used. A schematic view of an example of such a wind turbine is shown in FIG. 1.
It can occur that such a wind turbine starts oscillating and that these oscillations even affect the tower, which supports the aerodynamic rotor of the wind turbine. Generally, a differentiation is made between two types of oscillation, namely longitudinal oscillations and transverse oscillations, which, in principle, can also cancel each other out.
Essentially, transverse oscillation is oscillation that occurs transversely to the orientation of the rotor axis. Often, such transverse oscillation is caused by an imbalance of the rotor and thus oscillates correspondingly with the rotational speed of the rotor. Since, during rotation, this rotational imbalance directly causes these oscillations, they can also be referred to as forced oscillations.
Essentially, longitudinal oscillation occurs in the longitudinal direction of the rotation axis of the rotor. Thus, the wind turbine oscillates back and forth so to speak. In most cases, such longitudinal oscillations are caused by the wind and regularly interact with properties of the wind turbine. In particular, the oscillation frequency of such longitudinal oscillation orients itself on a natural frequency or a resonance frequency—which are usually very similar to each other—or on many such frequencies. Especially if the wind itself is volatile it can initiate such a longitudinal oscillation. Longitudinal oscillation can also be caused or increased by the fact that, through its control system, the wind turbine reacts to an increase in wind, including gusts of wind, and thus influences the motion of the wind turbine, wherein, in response to this regulating action, the reaction of the wind turbine leads to another change, which, in the worst case, can lead to oscillation.
A possible regulating action that could cause or increase such oscillation is the so-called pitch control, where the rotor blade angle is adjusted by a respective adjustment drive. Thus, increased wind pressure, for example, can be reduced by the respective adjustment of the rotor blades, through which, in turn, the wind pressure might become too weak so that the rotor blades have to be adjusted back until the wind pressure possibly becomes too high again, causing the pitch control to react once again, thereby possibly causing oscillating behavior.
With regard to the control system, such a problem could, of course, be taken into account, for example by setting up appropriate damping or by using more complex regulators, including, for example, a disturbance observer for an improved ability to take the disturbance into account. In this context, there is however the problem that such a change in the regulation structure may regularly have unforeseeable consequences. In addition, each wind turbine is individual and reacts individually. Naturally, one of the reasons for this is that each wind turbine is installed at a different location and that, therefore, conditions are never identical.
Another problem, which the present invention is also based on and which might have not been noticed in prior art, can occur in the case of several wind turbines impacting each other. In particular, in such a case, one can observe a phenomenon where a first wind turbine in a specific operating status is located, in particular in case of a certain wind direction, in the wind shadow behind a second wind turbine. In addition to the known energy deficits of the wind turbine in the wind shadow, oscillations can be transferred from the wind turbine in the front, i.e., the second wind turbine, to the wind turbine in the back, i.e., the first wind turbine, or may only be triggered in the wind turbine in the back.
The problem with such influences is, in particular, that they rarely occur and that it is therefore hard or even impossible to study them. Finally, the type of phenomenon being described can only occur if the wind direction is such that the first wind turbine is located behind the second wind turbine. However, even then such influences do not occur in principle but instead depend on other conditions, such as the prevailing wind speed or may be the gustiness of the prevailing wind.
The German Patent and Trade Mark Office researched the following prior art in the priority application: DE 10 2006 001 613 A1, DE 10 2008 009 740 A1, DE 10 2009 039 340 A1, DE 699 01 876 T2, US 2009/0200804 A1, WO 2007/089136 A2 and WO 2012/125842 A2.